1. Field of the Invention
The present invention relates to an optical head apparatus in which data may be stored on and read from an optical storage media, and more particularly to an improvement of an optical system thereof in its detection characteristics, which system employs a so-called twin-spot method as a track displacement detection technique for scanning data tracks of such optical storage media.
2. Description of the Prior Art
A conventional optical head apparatus is shown in FIGS. 13 (a), 13 (b) and 13 (c), in which: the reference numeral 1 denotes a semiconductor laser chip (LD chip serving as a light source; 5 an emitting light of the LD chip 1; 11 diffraction grating; 12 a beam splitter serving as a splitting means of the light; 13 an objective lens; 14 an optical disk constituting an optical data storage media with which the data may be optically stored and read from; 15 a photodetector for receiving light beams reflected from the optical disk 14; and 64 data-tracks on the optical disk 14. An incident light processing means is constructed of the diffraction grating 11 and the objective lens 13. FIG. 14 shows an enlarged view of a base element 4A constructed of a mount 4 for holding the LD chip 1. In FIG. 14, the reference numeral 1 denotes a semiconductor laser chip comprising, for example such as hetero junction of GaAs or GaAlAs, which chip 1 is fixed on the mount 4 constituting the base element 4A so that a front end face 100 of the chip 1 is made flush with a front end face 300 of the mount 4.
Now, the above conventional optical head apparatus will be hereinbelow described with reference to FIGS. 13 (a), 13 (b) and 13 (c).
A light beam 5 emitted from the LD chip 1 passes through a diffraction grating 11 to be divided in three beams as shown in dotted lines, which three divided beams are focused to three beam spot lights 60, 61 and 62 on the optical disk, respectively. Among them, the spot light 60 is produced by a zeroth order diffracted beam, while remaining two spot lights 61 and 62 of equal intensity are first order diffracted beams. As shown in FIG. 13 (a), the spot lights 60, 61 and 62 are so arranged that a straight line 63 connecting each of the centers of these spot lights 61, 60 and 62 in this order are slightly inclined with respect to a longitudinal direction of the track 64. These spot lights 60, 61 and 62 are reflected from the disk 14 and pass again through the objective lens 13, and thereafter are turned at right angles by means of the beam splitter 12 towards the photodetector 15. As shown in FIG. 13 (b), the photodetector 15 is constructed of three photodetection elements of 15a, 15b and 15c serving as photodetection portions, of which the central photodetection element 15a receives the reflected beam of the zeroth order spot light 60 while the remaining photodetection elements 15b and 15c receive the reflected beams of the first order spot lights 62 and 61, respectively, so as to emit electric signals corresponding to the intensity of light detected. As is publicly known, the zeroth order spot light 60 is employed to read/write the data of the disk 14. When the data is read from the disk 14, output signals of the central photodetection element 15a is employed as disk data signals. As described in the above, since the first order spot lights 62 and 61 are arranged to be slightly inclined with respect to the track 64, the spot light 62 and 61 are not symmetrically reflected with each other when the disk 14 is displaced in a direction R perpendicular to the longitudinal direction of the track 64 as shown in FIG. 13 (c), so that the output signals of the photodetection elements 15b and 15c vary nonsymmetrically with each other. As a result, as is well known, it is possible to obtain S-shaped track displacement signals with respect to misalignment between the track 64 and the center of the spot light 60, from the variations of the output signals of the phtodetection elements 15b and 15c, which track displacement signals are employed as so-called tracking error signals of a tracking-servo system for compensating a position of the spot light 60 on the track 64 in order to precisely read/write the data, so that the spot light 60 is correctly positioned at a center of the track 64. The tracking-servo system mentioned above is not described herein in detail since the present invention does not relate thereto in essence.
Next, the LD chip 1 employed in the conventional optical head apparatus will be described in construction with reference to FIG. 14. The LD chip 1 is a device for emitting the laser beam 5 when electric current flows through its PN-junction plane 2 formed inside the LD chip 1. However, at this time, such current increases the temperature of the LD chip 1, which leads to the decrease of the output power of the laser beam 5. Consequently, in order to keep the output power of the laser beam 5 constant, it is necessary to increase the electric current supplied to the LD chip 1, as is well known. Such increase of the electric current exponentially shortens the service life of the LD chip 1. Hitherto, in order to facilitate heat removal of the LD chip 1, the same 1 is fixed to the mount 4 which has an extraordinary large volume relative to that of the LD chip 1 and made of good thermal conductive material such as silver and brass, so that the heat generated in the LD chip 1 is dispersed through the mount 4 to prevent the temperature of the LD chip 1 from increasing. Incidentally, the LD chip 1 is fixed to the mount 4 through relatively soft material such as gold solder and indium solder which prevents internal stresses from occuring in the LD chip 1. Typically, a thickness of the LD chip is within a range of about from 50 to 90 .mu.m while that of the mount 4 is about 2000 .mu.m. As shown in FIG. 14, the front end face 100 of the LD chip 1 and that 300 of the mount 4 are arranged in parallel to each other to facilitate their assembling.
In the conventional optical head apparatus having the above construction, the misalignment between the track 64 and the zeroth order spot light 60 is detected through differences in amount of light between the two first order spot lights 61 and 62 detected by the photodetector 15, and at this time, when the track 64 of the disk 14 is angularly displaced in either direction, the detection characteristics of the conventional apparatus varies in a direction J shown in FIG. 13 (a). This is a defect inherent in the conventional apparatus. On the other hand, when the LD chip 1 is displaced in the same direction J, the detection characteristics of the conventional apparatus varies as is in the above case. This is also a defect inherent in the conventional apparatus. These defects lead to a problem that the tracking error signals for correctly scanning the track 64 are not adequately detected.
The problem will be hereinbelow described in detail with reference to FIGS. 13 (a), 13 (b), 13 (c) and 15.
As shown in FIG. 13 (a), 13(b) and 13(c), the three spot lights 60, 61 and 62, having been focused on the disk 14, are reflected, and then directed towards the photodetector 15 through the beam splitter 12 while parts of these spot lights 60, 61 and 62 pass again through the beam splitter 12 so as to return to the front end face 100 of the LD chip 1 through the diffraction grating 11 to be focused to three conjugate spot lights 50, 51 and 52 with respect to the spot lights 60, 61 and 62, respectively. At this time, typically, the spot lights 50, 51 and 52 are so arranged that they are spaced apart from each other at intervals of about 110 to 200 .mu.m, so that there is no fear that the spot light 51, having returned to a position above the LD chip 1, is again reflected. In contrast with this, the central spot light 50 is again reflected from the beam emitting position of the LD chip 1, while the lower first order spot light 62 is again reflected from the front end face 300 of the mount 4, towards the disk 14. FIG. 15 is a partially enlarged view of the conventional apparatus shown in FIG. 13 (a), in which the spot light 50 is one conjugate to the spot light 60 while the spot light 52 is one conjugate to the spot light 62. The lights having returned to the side of the LD chip 1 are again reflected from points "A" and D. FIG. 15 shows a case that the front end face 100 is parallel to the disk 14. In this case, strictly speaking, there is a difference .DELTA. 1 in distance between the spot light 52 and the reflecting end face of the LD chip 1, while a difference .DELTA. 2 between the spot light 62 and a light focusing point C as shown in FIG. 15. The original first order diffracted beams take light paths of AGC or AGE in FIG. 15, while the light having been reflected from the disk 14 and returned to the points "A" and D on the LD chip 1 as described above is, having been again reflected from the end face of the LD chip 1, again directed to the side of the disk 14. A light beam, having been reflected again, incident to the point C may take the following three light paths, provided that the first order diffraction of such incident beam takes place only one time:
a first beam 1' passing through a light path of AGBGAGC:
a second beam 2' passing through a light path of AGBGDGC; and
a third beam 3' passing through a light path of AGCGDGC. A light beam, which will be reflected again from the front end face 100 of the LD chip 1 toward a point E of the disk 14, may take the following only one light path since there is no reflection from the side of a point F shown in FIG. 15:
a fourth beam 4' passing through a light path of AGBGAGE. Now, in case that the disk 14 or both the LD chip 1 and the mount 4 are inclined in the direction J as shown in FIG. 15, a coherence length of the LD chip 1 is not so large that interference takes place between the beam passing through a light path of AGC, which becomes directly the first order diffracted beam, and the above first 1', second 2' and third 3' beams, and further between the beam passing through a light path of AGE and the above fourth beam 4'. In contrast with this, interference takes place among the beams 1', 2' and 3' which are incident again on the point C. In brief, as shown in FIG. 15, tghe differences in length of the light paths are: 2.DELTA.1 between the beams 1' 2'; 2.DELTA.2 between the beams 2' and 3'; and 2.DELTA.1+2.DELTA.2 between the beams 3' and 1', so that a variation of .DELTA.2 is produced when the disk 14 is finely angularly displaced in the direction J while a variation of .DELTA.1 is produced when the LD chip 1 is finely angularly displaced in the same direction J, whereby the above three differences in length of the light paths vary to produce phase differences among these light beams 1', 2' and 3' so that the intensity of the spot light on the point C of the disk 14 is varied by interference among these beams when the disk 14 or the LD chip 1 is rotatably driven. On the other hand, in the side of the point E of the disk 14, since the beam 4' is only one beam to be again reflected toward the point E, there is no interference in contrast with the side of the point C described above. The spot lights 61 and 62, having been produced by the beams diffracted through the diffraction grating 11, are equal in intensity to each other as already described in the above. Consequently, in FIG. 13, in case that there is no interference and that alignment between the center of the central spot light 60 and that of the track 64 is precisely established, the spot lights 61 and 62 are arranged on the track 64 with misalignment equal in amount and opposite in direction as shown in FIG. 13 (c), so that the resultant reflected beams from these spot light 61 and 62 are equal in intensity to each other, whereby the outputs of the photodetection elements 15b and 15c of the photodetector 15 are balanced against each other to go to zero. Under such condition, when the track 64 is displaced laterally, the difference between the outputs of the photodetection elements 15b and 15c varies to be plus or minus to make it possible to obtain a correct detection signal of the track's displacement, a cycle of which detection signal corresponds to a track's interval as shown in FIG. 16 (a) since the tracks 64 are arranged in cyclic manner while track's alignment points cross zero-level as shown in FIG. 16 (a). In contrast with this, in case that the above-mentioned interference is produced due to the inclination of the disk 14 so that the intensity of the light beam incident on, for example the photodetection element 15b becomes higher than that of the light beam incident on the photodetection element 15c, the detection signal of the track's displacement are so varied that the track's alignment points are spaced upward apart from the zero-level to the extent of "b" as shown in FIG. 16 (i), which deteriorates the detection characteristics of the conventional apparatus. In case that the LD chip 1 is inclined, the same deterioration occurs in the conventional apparatus.
In the conventional apparatus, for example, in case that: the spot lights' interval on the disk 14 is 22 .mu.m and the spot lights' interval on the LD chip 1 is 110 .mu.m, it is found by calculation that floating rate "b/a" (a half of the "a" is the amplitude of the detection signal of the track's displacement) of the track's alignment point varies in an 1.sup.. cycle with respect to the inclination of ".theta.D" of the disk 14 as shown in FIG. 17 (a). FIG. 17 (b) shows a condition in which the LD chip 1 is finely inclined by 0.02.sup.. from the condition shown in FIG. 17 (a), demonstrating that the rate "b/a" varies according to the inclination of the LD chip 1. The floating rate "b/a" varies in about a 0.02.sup.. cycle with respect to the inclination of the LD chip 1. In FIG. 15, in case that: reflectivity of the disk 14 is 80%; reflectivity of the front end face of the LD chip 1 is 30% (GaAs' reflectivity); reflectivity of the front end face of the mount 4 is 30%; diffraction rate of the zeroth order and the first order diffraction beams through the diffraction grating 11 is 5:1; and transmissivity of the beam splitter 12 is 50%, it is found by calculation that the floating rate "b/a" of the track's displacement reaches a maximum of about 26%, and this fact is confirmed from experiments. Consequently, due to such characteristics' variation, in the conventional apparatus, there is a fear that the detection characteristics of the track's alignment is considerably varied through the angular misalignment of the disk 14 occurred when the optical head apparatus is employed in the optical disk system and also through the angular misalignment of the LD chip 1 which may occur with age, to make it difficult to precisely read/write the data by the use of the conventional optical head apparatus.
As shown in FIGS. 18 to 20, it is also proposed to mount a buffer layer 3 on the mount 4 to construct the base element 4A so that the LD chip 1 is fixed to the buffer layer 3 of the base element 4A. The buffer layer 3 is employed for the following reason: namely, in case that the LD chip 1 is directly fixed to the mount 4, a large internal stress is produced in the LD chip 1 under the influence of heat generated in the chip 1 when the chip 1 is driven since the LD chip 1 and the mount 4 are considerably different in thermal expansion rate with each other. It is well known that such large internal stress of the LD chip 1 shortens the service life of the chip 1. In order to resolve this problem, the buffer layer 3 made of material provided with a thermal expansion rate similar to that of the LD chip 1, for example such as silicon is mounted on the mount 4 to construct the base element 4A on which the LD chip is mounted.
In such construction of the LD chip 1 and the buffer layer 36, the problems similar to those in the case shown in FIGS. 13 to 17 also occur as follows: namely, as shown in FIGS. 18 to 20, when the LD chip 1 is fixed to the mount 4 through the buffer layer 3 so that each of front end faces 100, 200 and 300 of the chip 1, the buffer layer 3 and the mount 4 are made parallel to each other, as shown in FIGS. 18(a), 18(b) and 18(c) the three spot lights 60, 61 and 62, having been focused on the disk 14, are reflected, and then directed towards the photodetector 15 through the beam splitter 12 while parts of these spot lights 60, 61 and 62 pass again through the beam splitter 12 so as to return to the front end face 100 of the LD chip 1 through the diffraction grating 11 to be focused to three conjugate spot lights 50, 51 and 52 with respect to the spot lights 60, 61 and 62, respectively. At this time, typically, the spot lights 50, 51 and 52 are so arranged that they are spaced apart from each other at intervals of about 110 to 200 .mu.m, so that there is no fear that the spot light 51, having returned to a position above the LD chip 1, is again reflected. In contrast with this, the central spot light 50 is again reflected from the beam emitting position of the LD chip 1, while the lower first order spot light 62 is again reflected from the front end face 200 of the buffer layer 3, towards the disk 14. FIG. 20 is a partially enlarged view of the conventional apparatus shown in FIG. 18(a), in which the spot light 50 is one conjugate to the spot light 60 while the spot light 52 is one conjugate to the spot light 62. The lights having returned to the side of the LD chip 1 are again reflected from points "A" and D. FIG. 20 shows a case that the front end face 100 is parallel to the disk 14. In this case, strictly speaking, there is a difference .DELTA.1 in distance between the spot light 52 and the reflecting end face of the buffer layer 3, while a difference .DELTA.2 between the spot light 62 and a light focusing point C as shown in FIG. 20. The original first order diffracted beams take light paths of AGC or AGE in FIG. 20, while the light having been reflected from the disk 14 and returned to the points "A" and D on the LD chip 1 as described above is, having been again reflected from the end face of the LD chip 1 and buffer layer 3, respectively, again directed to the side of the disk 14. A light beam, having been reflected again, incident to the point C may take the following three light paths, provided that the first order diffraction of such incident beam takes place only onetime: diffraction of such incident beam takes place only onetime:
a first beam 1' passing through a light path of AGBGAGC:
a second beam 2' passing through a light path of AGBGDGC; and
a third beam 3' passing through a light path of AGCGDGC. A light beam, which will be reflected again from the front end face 100 of the LD chip 1 toward a point E of the disk 14, may take the following only one light path since there is no reflection from the side of a point F shown in FIG. 15:
a fourth beam 4' passing through a light path of AGBGAGE. Now, in case that the disk 14 or the LD chip 1 and the buffer layer 3 are inclined in the direction J as shown in FIG. 20, a coherence length of the LD chip 1 is not so large that interference takes place between the beam passing through a light path of AGC, which becomes directly the first order diffracted beam, and the above first 1; second 2' and third 3' beams, and further between the beam passing through a light path of AGE and the above fourth beam 4'. In contrast with this, interference takes place among the beams 1', 2' and 3' which are incident again on the point C. In brief, as shown in FIG. 20, the differences in length of the light paths are: 2.DELTA.1 between the beams 1' and 2'; 2.DELTA.2 between the beams 2' and 3'; and 2.DELTA.1+2.DELTA.2 between the beams 3' and 1', so that a variation of .DELTA.2 is produced when the disk 14 is finely angularly displaced in the direction J while a variation of .DELTA.1 is produced when the LD chip 1 is finely angularly displaced in the same direction J, whereby the above three differences in length of the light paths vary to produce phase differences among these light beams 1', 2' and 3' so that the intensity of the spot light on the point C of the disk 14 is varied by interference among these beams when the disk 14 or the LD chip 1 is rotatably driven. On the other hand, in the side of the point E of the disk 14, since the beam 4' is only one beam to be again reflected toward the point E, there is no interference in contrast with the side of the point C described above. The spot lights 61 and 62, having been produced by the beams diffracted through the diffraction grating 11, are equal in intensity to each other as already described in the above. Consequently, in FIG. 18, in case that there is no interference and that alignment between the center of the central spot light 60 and that of the track 64 is precisely established, the spot lights 61 and 62 are arranged on the track 64 with misalignment equal in amount and opposite in direction as shown in FIG. 18(c), so that the resultant reflected beams from these spot lights 61 and 62 are equal in intensity to each other. whereby the outputs of the photodetection elements 15b and 15c of the photodetector 15 are balanced against each other to go to zero. Under such condition, when the track 64 is displaced laterally, the difference between the outputs of the photodetection elements 15b and 15c varies to be plus or minus to make it possible to obtain a correct detection signal of the track's displacement, a cycle of which detection signal corresponds to a track's interval as shown in FIG. 16 (a) since the tracks 64 are arranged in cyclic manner while track's alignment points cross zero-level as shown in FIG. 16(a). In contrast with this, in case that the above-mentioned interference is produced due to the inclination of the disk 14 so that the intensity of the light beam incident on, for example the photodetection element 15b becomes higher than that of the light beam incident on the photodetection element 15c, the detection signal of the track's displacement are so varied that the track's alignment points are spaced upward apart from the zero-level to the extent of "b" as shown in FIG. 16 (b), which deteriorates the detection characteristics of the conventional apparatus. In case that the LD chip 1 is inclined, the same deterioration occurs in the conventional apparatus.
In the conventional apparatus, for example, in case that: the spot lights' interval on the disk 14 is 22.mu.m and the spot lights' interval on the LD chip 1 is 110 .mu.m, it is found by calculation that floating rate "b/a" (a half of the "a" is the amplitude of the detection signal of the track's displacement) of the track's alignment point varies in an 1.sup.. cycle with respect to the inclination of ".theta.D" of the disk 14 as shown in FIG. 17 (a). FIG. 17 (b) shows a condition in which the LD chip 1 is finely inclined by 0.02.sup.. from the condition shown in FIG. 17 (a), demonstrating that the rate "b/a" varies according to the inclination of the LD chip 1. The floating rate "b/a" varies in about 0.2.sup.. cycle with respect to the inclination of the LD chip 1. In FIG. 20, in case that: reflectivity of the disk 14 is 80% reflectivity of the front end face of the LD chip 1 is 30% (GaAs' reflectivity); reflectivity of the front end face of the buffer layer 3 is 30% (Si's reflectivity; diffraction rate of the zeroth order and first order diffracted beams through the diffraction grating 11 is 5:1; and transmissivity of the beam splitter 12 is 50%, it is found by calculation that the floating rate "b/a" of the of the track's displacement reaches a maximum of about 26%, and this fact is confirmed from experiments. Consequently, due to such characteristics' variation, in the conventional apparatus, there is a fear that the detection characteristics of the track's alignment is considerably varied through the angular misalignment of the disk 14 occured when the optical head apparatus is employed in the optical disk system and also through the angular misalignment of the LD chip 1 which may occur with age, to make it difficult to precisely read/write the data by the use of the conventional optical head apparatus.